The present invention relates to a method for triggering an alarm in an infusion syringe before the syringe is emptied of infusion liquid.
The use of a syringe pump which continuously forces a liquid such as a drug out of an infusion syringe for injection into the body of a patient is known. Such syringe pumps comprise a holder to which a syringe cylinder is attached and a slide movable relative to the holder, the slide being connected to a syringe piston movable in the syringe cylinder. The slide is moved continuously by a motor drive, so that advance of the syringe piston by the slide forces the injection liquid out of the syringe.
In order to indicate to an attending physician or nurse that the syringe is almost empty, an alarm is produced before the infusion syringe is emptied of infusion liquid. Producing an alarm before the syringe is completely emptied enables the appropriate preparatory measures to be taken, for example, those necessary to replace the near-empty syringe with a new full syringe. In providing such an alarm, known syringe pumps utilize a sensor responsive to a predetermined position of the slide which corresponds to a predetermined position of the syringe piston before its end position.
However, infusion pumps are used not only in connection with a specific syringe size or type, but also with different size and type syringes. Also the rate of advance of the syringe pump slide is adjustable in some syringe pumps so as to be able to attain different rates of infusion, which for example can be in the range of from 0.1 to 99.9 ml/h. With such a wide range of infusion rates, the time which elapses between the activation of the sensor and complete evacuation of the syringe can vary greatly. Normally the sensor is positioned so that at the maximum infusion rate, three minutes elapse between activation of the sensor and emptying of the syringe. If the infusion rate is lower (slower) than the maximum infusion rate, the alarm will be triggered relatively early, so that it does not effectively indicate the time at which the syringe is almost empty. An alarm which is not in a fixed-time relationship with the event it is to announce is practically useless and can even be misleading.